The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Police have picture of NZ tourist's killer

Argentinian police are preparing an identikit picture of the man who shot New Zealand tourist Nick Heyward during a botched robbery while he strolled through a park in the city of Mendoza.

Newspapers in the popular tourist destination report that witnesses have given statements to police about the attack by two motorcycle-riding robbers, known as "motochorros", who tried to snatch Mr Heyward's backpack on Monday (local time).

Although there were conflicting accounts of whether the motochorros had helmets covering their faces, one witness told police the man who shot Mr Heyward was not wearing a helmet - enabling police to produce a sketch.

All witnesses agreed the motochorros were aged about 20.

The newspapers report that police have high hopes of finding the offenders and although images from security cameras are not "great" they might help.

Mr Heyward, 31, was shot dead after he resisted the robbers' bag snatch attempt in General San Martin Park where he was talking with fellow tourists, an Australian woman and an French man.

Mr Heyward was shot up to four times between chest and neck with a .22 pistol, police say.

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He had been travelling after finishing a degree in physiotherapy at the University of Queensland and been in Central America for about a year. But he was planning to return in a few months to Australia, where he has family in Adelaide and on the Gold Coast.

Arrangements are being made for his body to be returned and one family member is expected to travel to Mendoza in the next few days.

The editor of Buenos Aires-based Argentina Independent, Marc Rogers, says Mendoza is a popular tourist destination because of its wineries and the nearby Andes.

Tourists were always at risk of being robbed but it generally didn't come with that level of violence, he told RadioLive.

However, motorcycle bag snatchers were now a more common style of crime - and Argentinian authorities were trying new laws to combat it.

Argentina's Ministry of Security has offered 40,000 pesos ($A5,446) to help find the offenders.

Mendoza newspapers report there will be a protest in General San Martin Park on Saturday morning local time to press for more security in the park.